1. Field of the Invention
The present invention relates to a magnesium additive device and, more particularly, to a method of adding high vapor pressure magnesium to a steel liquid and an apparatus for performing the method.
2. Description of the Related Art
Steelmaking is an oxidation refining process, namely, to blow oxygen into a hot metal, to remove excessive elements (C, Si, Mn) and impurities (S, P) contained in the ingot iron. The maximum oxygen content in the steel liquid is up to 0.1% during the final steelmaking stage by the effect of oxygen blowing steelmaking. However, the solubility of oxygen in the solid steel is very low (e.g., maximum solubility in δ-Fe is 0.0082%). The excessive oxygen is in the form of FeO or other oxides in the process of solidification, thereby reducing the performance of the steel. Therefore, it is necessary to remove the oxygen from the steel liquid at the end of decarbonization. The oxygen in the steel includes [O]D and [O]L. Deoxidization is to add a deoxidant (such as manganese, silicon, titanium, aluminum, magnesium, etc.) to the liquid steel, so as to reduce the oxygen content in the steel. In general, the total oxygen content T[O] in the steel represents the cleanliness of the steel liquid.
The oxygen content in the steel is closely related to the quality of the product. FIG. 1 is a table showing an oxygen content demand of a conventional steel product. Thus, it is important to reduce the oxygen content in the steel so as to enhance the cleanliness purity of the steel.
The alloys having a deoxidizing capacity include a weak deoxidant (such as manganese, silicon, titanium, etc.) and a strong deoxidant (such as aluminum, magnesium, etc.). The weak deoxidant, such as manganese or silicon, has a poor deoxidizing capacity so that the oxygen content in the steel is high. The strong deoxidant, such as aluminum, has an excellent deoxidizing capacity so that the oxygen content in the steel is low. For example, studies have shown that, when the aluminum content dissolved in the steel is about 0.03% to 0.05%, the average oxygen content in the steel is reduced to 3 ppm or less. Thus, the aluminum is often used as a deoxidant in the steelmaking industry. However, after the aluminum deoxidization process, the cluster inclusion Al2O3 contained in the steel will greatly reduce the ductility, toughness, fatigue strength and corrosion resistance of the steel. Especially, the cluster inclusion Al2O3 contained in the steel undermines the continuity of the steel substrate, resulting in a material failure under actions of static load and dynamic load, to reduce the steel production yield and the product quality.
The magnesium has a strong chemical activity so that Mg in the steel liquid has a strong affinity for the non-metallic elements. Thus, it is possible to use the magnesium as a deoxidant. Tateyama used a cored wire made of magnesium (Mg, MgO, CaF2) to perform deoxidization and desulfurization experiments in a reaction furnace that is protected by a low-carbon steel argon. The adding amount of magnesium is less than 15%. The T[O] value in the liquid steel is down to 9 ppm and the [S] value in the liquid steel is down to 3 ppm after five minutes. The T[O] and [S] values are not raised after ten minutes and are kept at 11 ppm and 3 ppm respectively. Therefore, it can be seen that, Mg has a strong affinity with S and O in the steel liquid, without changing the composition of the steel liquid, so that Mg is an ideal deoxidant and desulfurizer.
In fact, in the steel production process, the gas injection method is used to inject the passivation magnesium particles into the hot metal to achieve the purpose of desulfurization of the hot metal. A similar method is used to add the magnesium to the hot metal to produce a nodular cast iron. FIG. 2 is a graph showing the relationship of the vapor pressure and the temperature of Mg and Al. As shown in FIG. 2, Mg has a very high vapor pressure (200 times of that of aluminum) under the steelmaking temperature (1600° C.). Obviously, the steel liquid temperature (1600° C.) is much higher that the hot metal temperature (1300-1400° C.), so that the gas injection method of injecting the passivation magnesium particles cannot be directly used to the magnesium procedure in the steel liquid. Thus, the important core of the magnesium procedure in the steel liquid is how to add magnesium into the steel liquid safely with a high gain under the steelmaking temperature of 1600° C.
The conventional methods of adding the magnesium into the steel liquid include a pouring method, a cored wire method and a plunging method. The pouring method is to pour the magnesium particles from the steel liquid into a ladle. The added alloys include rare earth magnesium alloy, high magnesium alloy and passivated magnesium particles.
Another special structure includes a cored wire injection system which feeds a magnesium cored wire deeply into the steel liquid. Obviously, when the magnesium cored wire reaches the bottom of the steel liquid, the phase change of the magnesium is as follows: Mg(s)→Mg(l)→Mg(g). A large temperature change occurs between the magnesium cored wire and the steel liquid, so that the process of Mg(s)→Mg(g) is finished in a very short period of time. Thus, a very large pressure is produced during the vaporized process of the magnesium, so that the steel liquid is easily stirred to cause accidents. An aluminothermic reduction method is used to produce a Mg vapor, with the argon functioning as a carrier to introduce the Mg vapor into a steel liquid.